A coating composition containing as essential components a binder composition comprising a hydroxyl bearing heat curable resin and an aminoaldehyde resin and/or isocyanate compound, and a pigment has been widely used in various fields because of its capability of giving a cross-linked tough coating through baking. However, depending on the kind of base resin used, the coating performance, the application characteristics, the appearance and the like are divided in every case, and with the diversification in needs, further improvements in film performance and especially weathering resistance have been longed for.
As the resinous binder for coating compositions, a combination of oil-free polyester and aminoaldehyde resins has been well known and watched with keen interest because they have improved properties such as weathering resistance, improved mechanical properties when used as a coating, improved interlaminar adhesion and the like. However, since there is a trend of these coatings of to easily generate craters and to lose their gloss unevenly and since a polyester has relatively poor compatibility with amino resins, such combination has not been used to widely. Recently, many important facts have been found out such that by the use of saturated alicyclic polybasic acids or the combination of saturated alicyclic polybasic acids and aromatic polybasic acids as a part of the acid component of the oil-free polyester resin, considerable improvements in application characteristics, uneven such as, compatibility with amino resins and improved gloss and curing properties can be attained, and that by the modification of the polyester resin with a reactive organopolysiloxane resin the, weathering resistance can be improved, and that by reacting the polyester resin with a saturated alicyclic polybasic acid and then subjecting it to a silicon modification, further improvements in application characteristics (pinhole, sagging and cratering resistances), gloss and weathering resistance can be obtained, and on the basis of these findings, polyester based coating compositions have suddenly become the object of public attention as top-coats for automobile coil coatings and the like. However, when a polyester resin is subjected to a silicon modification, an improvement in weathering resistance is indeed realized but an additional problem of decreased recoatability occurs. Therefore, in order to make a polyester base composition which is widely useful as a top-coat, it is essential that the recoatability as well as the weathering resistance thereof be improved. Various proposals heretofore made have given enough answers to the above problems.
The inventors, with the object of improving both the recoatability and the weathering resistance of a polyester resin based composition, have endeavored and found that an improvement in the weathering resistance of a polyester composition is most advantageously achieved by the abovesaid silicon modification and the loss of recoatability in such composition can be effectively cured by the introduction of particular alkylene imine moieties thereto, and that such means can successfully be applied to other resins than polyesters, as well. As the basis of these findings, the invention has been made.